Power factor correction with variable bus voltage

ABSTRACT

A controller includes a voltage determination module, a bus voltage command module, and a power factor correction (PFC) control module. The voltage determination module determines a desired direct current (DC) bus voltage for a DC bus electrically connected between a PFC module and an inverter power module that drives a motor. The voltage determination module determines the desired DC bus voltage based on at least one of torque of the motor and speed of the motor. The bus voltage command module determines a commanded bus voltage based on the desired DC bus voltage. The PFC control module controls the PFC module to create a voltage on the DC bus that is based on the commanded bus voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims is a continuation of U.S. application Ser. No.13/964,595, filed on Aug. 12, 2013, which is a continuation of U.S.application Ser. No. 12/852,557, filed on Aug. 9, 2010, which claims thebenefit of U.S. Provisional App. No. 61/232,754, filed on Aug. 10, 2009.The entire disclosures of the above applications are incorporated hereinby reference.

FIELD

The present disclosure relates to electric motor control systems andmethods and more particularly to power factor correction systems andmethods.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Electric motors are used in a wide variety of industrial and residentialapplications including, but not limited to, heating, ventilating, andair conditioning (HVAC) systems. For example only, an electric motor maydrive a compressor in an HVAC system. One or more additional electricmotors may also be implemented in the HVAC system. For example only, theHVAC system may include another electric motor that drives a fanassociated with a condenser. Another electric motor may be included inthe HVAC system to drive a fan associated with an evaporator.

Power factor is an indicator of the relationship between current andvoltage in a circuit, or how effectively a circuit uses real powercompared to storing and returning energy to the power source. Powerfactor may be expressed as a value between zero and one. The circuit'suse of actual real power divided by the total volt amps drawn by thecircuit may increase as the power factor approaches one. In variousimplementations, a power factor correction (PFC) system may beimplemented. PFC systems generally operate to increase a circuit's powerfactor toward one, thereby increasing the circuit's use of real power ascompared with the amount of reactive power the circuit stores andreturns to the source.

SUMMARY

A system includes a power factor correction (PFC) module, an inverterpower module, and a controller. The PFC module converts incoming ACpower into DC power. The inverter power module converts the DC powerinto three-phase AC power and drives a motor of a compressor using thethree-phase AC power. The controller includes a voltage determinationmodule, a voltage command module, a rate limiting module, and a PFCcontrol module. The voltage determination module determines a desiredvoltage for the DC power based on at least one of a plurality of systemparameters.

The voltage command module generates a commanded voltage based on thedesired voltage. The voltage command module sets the commanded voltageequal to a startup voltage for a predetermined startup period after thecontroller is powered on. After the predetermined startup period, thevoltage command module performs three functions. First, the voltagecommand module increases the commanded voltage to the desired voltagewhen the desired voltage is greater than the commanded voltage.

Second, the voltage command module increases the commanded voltage to afirst threshold voltage when the first threshold voltage is greater thanthe commanded voltage. The first threshold voltage is based on a sum ofa predetermined positive offset voltage and a measured peak voltage ofthe incoming AC power. Third, the voltage command module selectivelydecreases the commanded voltage to a greater one of a second thresholdvoltage and the desired voltage after a predetermined period has elapsedin which the commanded voltage has not been increased. The secondthreshold voltage is based on a sum of the offset voltage and a highestvalue of the measured peak voltage of the incoming AC power observedthroughout the predetermined period.

The rate limiting module generates a limited commanded voltage bylimiting a rate of change of the commanded voltage. When the controlleris powered on, the rate limiting module initializes the limitedcommanded voltage to a measured voltage of the DC power. The PFC controlmodule controls the PFC module to produce the DC power at the limitedcommanded voltage. In other features, the system further includes thecompressor. The plurality of system parameters includes torque of themotor, speed of the motor, output power of the inverter power module,and drive input power.

A controller includes a voltage determination module, a bus voltagecommand module, and a power factor correction (PFC) control module. Thevoltage determination module determines a desired direct current (DC)bus voltage for a DC bus electrically connected between a PFC module andan inverter power module that drives a compressor motor. The voltagedetermination module determines the desired DC bus voltage based on atleast one of torque of the compressor motor, speed of the compressormotor, output power of the inverter power module, and drive input power.The bus voltage command module determines a commanded bus voltage basedon the desired DC bus voltage. The PFC control module controls the PFCmodule to create a voltage on the DC bus that is based on the commandedbus voltage.

In other features, the bus voltage command module sets the commanded busvoltage equal to a measured voltage of the DC bus when the controllertransitions from an off state to an on state.

In further features, the controller further includes a rate limitingmodule that generates a rate limited voltage. The PFC control modulecontrols the PFC module to create the rate limited voltage on the DCbus. The rate limited voltage is equal to the measured voltage of the DCbus when the controller transitions from the off state to the on state.After the controller transitions from the off state to the on state, thebus voltage command module sets the commanded bus voltage equal to apredetermined startup voltage for a predetermined startup period, andthe rate limiting module ramps the rate limited voltage toward thecommanded bus voltage during the predetermined startup period.

In still other features, the bus voltage command module increases thecommanded bus voltage to a greater one of the desired DC bus voltage anda first sum when the commanded bus voltage is less than either thedesired DC bus voltage or the first sum. The first sum is equal to apredetermined offset plus a peak voltage of an AC line powering the PFCmodule.

In other features, the bus voltage command module decreases thecommanded bus voltage to a greater one of the desired DC bus voltage anda second sum after a predetermined period in which the commanded busvoltage was not increased. The second sum is equal to the predeterminedoffset plus a highest value of the peak voltage observed during thepredetermined period.

In further features, a system includes the controller, the PFC module,the inverter power module, and a condenser inverter module that drives acondenser fan using power from the DC bus. A system includes thecontroller, the PFC module, the inverter power module, a condenserinverter module that drives a condenser fan using power from a second DCbus, and an electrical linkage between the DC bus and the second DC busthat provides excess power from the DC bus to the second DC bus.

A method includes converting incoming AC power into DC power using apower factor correction (PFC) module; converting the DC power into ACpower using an inverter power module; driving a motor of a compressorusing the AC power; determining a desired voltage for the DC power basedon at least one of torque of the motor, speed of the motor, output powerof the inverter power module, and drive input power; generating acommanded voltage based on the desired voltage; and controlling the PFCmodule to produce the DC power at a voltage based on the commandedvoltage.

In other features, the method further includes setting the commandedvoltage equal to a startup voltage for a predetermined startup periodupon power-on.

In further features, the method further includes generating a limitedcommanded voltage by limiting a rate of change of the commanded voltage;controlling the PFC module to produce the DC power at the limitedcommanded voltage; and at a beginning of the predetermined startupperiod, initializing the limited commanded voltage to a measured voltageof the DC power.

In still other features, the method further includes maintaining thecommanded voltage to be greater than or equal to the desired voltage.The method further includes determining a threshold voltage based on asum of a predetermined positive offset voltage and a measured peakvoltage of the incoming AC power; and maintaining the commanded voltageto be greater than or equal to the threshold voltage.

In other features, the method further includes increasing the commandedvoltage to the desired voltage when the desired voltage is greater thanthe commanded voltage; and increasing the commanded voltage to a firstthreshold voltage when the first threshold voltage is greater than thecommanded voltage. The first threshold voltage is based on a sum of apredetermined positive offset voltage and a measured peak voltage of theincoming AC power.

In further features, the method further includes selectively decreasingthe commanded voltage to a greater one of a second threshold voltage andthe desired voltage after a predetermined period has elapsed in whichthe commanded voltage has not been increased. The second thresholdvoltage is based on a sum of the offset voltage and a highest value ofthe measured peak voltage of the incoming AC power observed throughoutthe predetermined period.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples areintended for purposes of illustration only and are not intended to limitthe scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of an example refrigeration system;

FIG. 2 is a functional block diagram of an example drive controller andan example compressor;

FIGS. 3 a-3 c are simplified schematics of example power factorcorrection (PFC) modules;

FIGS. 4 a-4 c are simplified schematics of example inverter powermodules and example motors;

FIG. 5 is a functional block diagram of an example implementation of acommon direct current (DC) bus refrigeration system;

FIG. 6 is a functional block diagram of another example implementationof a common DC bus refrigeration system;

FIG. 7 is a functional block diagram of an example bus voltagedetermination module; and

FIG. 8 is a flow diagram of example method for determining the DC busvoltage.

DETAILED DESCRIPTION

The following description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. Forpurposes of clarity, the same reference numbers will be used in thedrawings to identify similar elements. As used herein, the phrase atleast one of A, B, and C should be construed to mean a logical (A or Bor C), using a non-exclusive logical or. It should be understood thatsteps within a method may be executed in different order withoutaltering the principles of the present disclosure.

As used herein, the term module may refer to, be part of, or include anApplication Specific Integrated Circuit (ASIC); an electronic circuit; acombinational logic circuit; a field programmable gate array (FPGA); aprocessor (shared, dedicated, or group) that executes code; othersuitable components that provide the described functionality; or acombination of some or all of the above, such as in a system-on-chip.The term module may include memory (shared, dedicated, or group) thatstores code executed by the processor.

The term code, as used above, may include software, firmware, and/ormicrocode, and may refer to programs, routines, functions, classes,and/or objects. The term shared, as used above, means that some or allcode from multiple modules may be executed using a single (shared)processor. In addition, some or all code from multiple modules may bestored by a single (shared) memory. The term group, as used above, meansthat some or all code from a single module may be executed using a groupof processors. In addition, some or all code from a single module may bestored using a group of memories.

The apparatuses and methods described herein may be implemented by oneor more computer programs executed by one or more processors. Thecomputer programs include processor-executable instructions that arestored on a non-transitory tangible computer readable medium. Thecomputer programs may also include stored data. Non-limiting examples ofthe non-transitory tangible computer readable medium are nonvolatilememory, magnetic storage, and optical storage.

Referring now to FIG. 1, a functional block diagram of a refrigerationsystem 100 is presented. The refrigeration system 100 may include acompressor 102, a condenser 104, an expansion valve 106, and anevaporator 108. According to the principles of the present disclosure,the refrigeration system 100 may include additional and/or alternativecomponents. In addition, the present disclosure is applicable to othersuitable types of refrigeration systems including, but not limited to,heating, ventilating, and air conditioning (HVAC), heat pump,refrigeration, and chiller systems.

The compressor 102 receives refrigerant in vapor form and compresses therefrigerant. The compressor 102 provides pressurized refrigerant invapor form to the condenser 104. The compressor 102 includes an electricmotor that drives a pump. For example only, the pump of the compressor102 may include a scroll compressor and/or a reciprocating compressor.

All or a portion of the pressurized refrigerant is converted into liquidform within the condenser 104. The condenser 104 transfers heat awayfrom the refrigerant, thereby cooling the refrigerant. When therefrigerant vapor is cooled to a temperature that is less than asaturation temperature, the refrigerant transforms into a liquid (orliquefied) refrigerant. The condenser 104 may include an electric fanthat increases the rate of heat transfer away from the refrigerant.

The condenser 104 provides the refrigerant to the evaporator 108 via theexpansion valve 106. The expansion valve 106 controls the flow rate atwhich the refrigerant is supplied to the evaporator 108. The expansionvalve 106 may include a thermostatic expansion valve or may becontrolled electronically by, for example, a system controller 130. Apressure drop caused by the expansion valve 106 may cause a portion ofthe liquefied refrigerant to transform back into the vapor form. In thismanner, the evaporator 108 may receive a mixture of refrigerant vaporand liquefied refrigerant.

The refrigerant absorbs heat in the evaporator 108. Liquid refrigeranttransitions into vapor form when warmed to a temperature that is greaterthan the saturation temperature of the refrigerant. The evaporator 108may include an electric fan that increases the rate of heat transfer tothe refrigerant.

A utility 120 provides power to the refrigeration system 100. Forexample only, the utility 120 may provide single-phase alternatingcurrent (AC) power at approximately 230 Volts (V) root mean squared(RMS) or at another suitable voltage. In various implementations, theutility 120 may provide three-phase power at approximately 400 Volts RMSor 480 Volts RMS at a line frequency of, for example, 50 or 60 Hz. Theutility 120 may provide the AC power to the system controller 130 via anAC line. The AC power may also be provided to a drive controller 132 viathe AC line.

The system controller 130 controls the refrigeration system 100. Forexample only, the system controller 130 may control the refrigerationsystem 100 based on user inputs and/or parameters measured by varioussensors (not shown). The sensors may include pressure sensors,temperature sensors, current sensors, voltage sensors, etc. The sensorsmay also include feedback information from the drive control, such asmotor currents or torque, over a serial data bus or other suitable databuses.

A user interface 134 provides user inputs to the system controller 130.The user interface 134 may additionally or alternatively provide theuser inputs to the drive controller 132. The user inputs may include,for example, a desired temperature, requests regarding operation of afan (e.g., the evaporator fan), and/or other suitable inputs. The systemcontroller 130 may control operation of the fans of the condenser 104,the evaporator 108, and/or the expansion valve 106.

The drive controller 132 may control the compressor 102 based oncommands from the system controller 130. For example only, the systemcontroller 130 may instruct the drive controller 132 to operate thecompressor motor at a certain speed. In various implementations, thedrive controller 132 may also control the condenser fan.

Referring now to FIG. 2, a functional block diagram of the drivecontroller 132 and the compressor 102 is presented. An electromagneticinterference (EMI) filter 202 reduces EMI that might otherwise beinjected back onto the AC line by the drive controller 132. The EMIfilter 202 may also filter EMI carried on the AC line.

A power factor correction (PFC) module 204 receives AC power from the ACline as filtered by the EMI filter 202. The PFC module 204 (described inmore detail with reference to FIGS. 3 a, 3 b, and 3 c) rectifies the ACpower, thereby converting the AC input power into direct current (DC)power. The generated DC power is provided at positive and negativeterminals of the PFC module 204. The PFC module 204 also selectivelyprovides power factor correction between the input AC power and thegenerated DC power.

The PFC module 204 selectively boosts the AC power to a DC voltage thatis greater than a peak voltage of the AC power. For example only, thePFC module 204 may operate in a passive mode, where the DC voltagegenerated is less than a peak voltage of the AC power. The PFC module204 may also operate in an active mode, where the DC voltage generatedis greater than the peak voltage of the AC power. A DC voltage that isgreater than the peak voltage of the AC power may be referred to as aboosted DC voltage.

AC power having an RMS voltage of 230 V has a peak voltage ofapproximately 325 V (230 V multiplied by the square root of 2). Forexample only, when operating from AC power having an RMS voltage of 230V, the PFC module 204 may generate boosted DC voltages betweenapproximately 350 V and approximately 410 V. For example only, the lowerlimit of 350 V may be imposed to avoid unstable operating regimes of thePFC module 204. The limits may vary, such as with the actual AC inputvoltage value. In various implementations, the PFC module 204 may beable to achieve higher boosted DC voltages than 410 V. However, theupper limit may be imposed to improve long-term reliability ofcomponents that would experience greater stress at higher voltages, suchas components in a DC filter 206. In various implementations, the upperand/or lower limits may be varied.

The DC filter 206 filters the DC power generated by the PFC module 204.The DC filter 206 minimizes ripple voltage present in the DC power thatresults from the conversion of AC power to DC power. In variousimplementations, the DC filter 206 may include one or more series orparallel filter capacitors connected between the positive and negativeterminals of the PFC module 204. In such implementations, the positiveand negative terminals of the PFC module 204 may be connected directlyto positive and negative terminals of an inverter power module 208.

The inverter power module 208 (described in more detail with referenceto FIGS. 4 a, 4 b, and 4 c) converts the DC power, as filtered by the DCfilter 206, into AC power that is provided to the compressor motor. Forexample only, the inverter power module 208 may convert the DC powerinto three-phase AC power and provide the phases of the AC power tothree respective windings of the motor of the compressor 102. In otherimplementations, the inverter power module 208 may convert the DC powerinto more or fewer phases of power.

A DC-DC power supply 220 may also receive the filtered DC power. TheDC-DC power supply 220 converts the DC power into one or more DCvoltages that are suitable for various components and functions. Forexample only, the DC-DC power supply 220 may reduce the voltage of theDC power to a first DC voltage that is suitable for powering digitallogic and a second DC voltage that is suitable for controlling switcheswithin the PFC module 204. For example only, the second DC voltage maybe selectively applied to gate terminals of the switches. In variousimplementations, DC power may be provided by another DC power source(not shown)—for example, a DC voltage derived via a transformer from themain 230 VAC input.

In various implementations, the first DC voltage may be approximately3.3 V and the second DC voltage may be approximately 15 V. In variousimplementations, the DC-DC power supply 220 may also generate a third DCvoltage. For example only, the third DC voltage may be approximately 1.2V. The third DC voltage may be derived from the first DC voltage using avoltage regulator. For example only, the third DC voltage may be usedfor core digital logic and the first DC voltage may be used forinput/output circuitry of a PFC control module 250 and a motor controlmodule 260.

The PFC control module 250 controls the PFC module 204, and the motorcontrol module 260 controls the inverter power module 208. In variousimplementations, the PFC control module 250 controls switching of theswitches within the PFC module 204, and the motor control module 260controls switching of switches within the inverter power module 208. ThePFC module 204 may be implemented with 1, 2, 3, or more phases.

A supervisor control module 270 may communicate with the systemcontroller 130 via a communications module 272. The communicationsmodule 272 may include an input/output port and other suitablecomponents to serve as an interface between the system controller 130and the supervisor control module 270. The communications module 272 mayimplement wired and/or wireless protocols.

The supervisor control module 270 provides various commands to the PFCcontrol module 250 and the motor control module 260. For example, thesupervisor control module 270 may provide a commanded speed to the motorcontrol module 260. The commanded speed corresponds to a desiredrotational speed of the motor of the compressor 102.

In various implementations, the commanded compressor speed may beprovided to the supervisor control module 270 by the system controller130. In various implementations, the supervisor control module 270 maydetermine or adjust the commanded compressor speed based on inputsprovided via the communications module 272 and/or parameters measured byvarious sensors (i.e., sensor inputs). The supervisor control module 270may also adjust the commanded compressor speed based on feedback fromthe PFC control module 250 and/or the motor control module 260.

The supervisor control module 270 may also provide other commands to thePFC control module 250 and/or the motor control module 260. For example,based on the commanded speed, the supervisor control module 270 maycommand the PFC control module 250 to produce a commanded bus voltage.The supervisor control module 270 may adjust the commanded bus voltagebased on additional inputs, such as operating parameters of the inverterpower module 208 and the measured voltage of the incoming AC line.

The supervisor control module 270 may diagnose faults in various systemsof the drive controller 132. For example only, the supervisor controlmodule 270 may receive fault information from the PFC control module 250and/or the motor control module 260. The supervisor control module 270may also receive fault information via the communications module 272.The supervisor control module 270 may manage reporting and clearing offaults between the drive controller 132 and the system controller 130.

Responsive to the fault information, the supervisor control module 270may instruct the PFC control module 250 and/or the motor control module260 to enter a fault mode. For example only, in the fault mode, the PFCcontrol module 250 may halt switching of the switches of the PFC module204, while the motor control module 260 may halt switching of theswitches of the inverter power module 208. In addition, the motorcontrol module 260 may directly provide fault information to the PFCcontrol module 250. In this way, the PFC control module 250 can respondto a fault identified by the motor control module 260 even if thesupervisor control module 270 is not operating correctly and vice versa.

The PFC control module 250 may control switches in the PFC module 204using pulse width modulation (PWM). More specifically, the PFC controlmodule 250 may generate PWM signals that are applied to the switches ofthe PFC module 204. The duty cycle of the PWM signals is varied toproduce desired currents in the switches of the PFC module 204. Thedesired currents are calculated based on an error between the measuredDC bus voltage and a desired DC bus voltage. In other words, the desiredcurrents are calculated in order to achieve the desired DC bus voltage.The desired currents may also be based on achieving desired power factorcorrection parameters, such as the shapes of current waveforms in thePFC module 204. The PWM signals generated by the PFC control module 250may be referred to as PFC PWM signals.

The motor control module 260 may control switches in the inverter powermodule 208 using PWM in order to achieve the commanded compressor speed.The PWM signals generated by the motor control module 260 may bereferred to as inverter PWM signals. The duty cycle of the inverter PWMsignals controls the current through the windings of the motor (i.e.,motor currents) of the compressor 102. The motor currents control motortorque, and the motor control module 260 may control the motor torque toachieve the commanded compressor speed.

In addition to sharing fault information, the PFC control module 250 andthe motor control module 260 may also share data. For example only, thePFC control module 250 may receive data from the motor control module260 such as load, motor currents, estimated motor torque, invertertemperature, duty cycle of the inverter PWM signals, and other suitableparameters. The PFC control module 250 may also receive data from themotor control module 260, such as the measured DC bus voltage. The motorcontrol module 260 may receive data from the PFC control module 250 suchas AC line voltage, current(s) through the PFC module 204, estimated ACpower, PFC temperature, commanded bus voltage, and other suitableparameters.

In various implementations, some or all of the PFC control module 250,the motor control module 260, and the supervisor control module 270 maybe implemented on an integrated circuit (IC) 280. For example only, theIC 280 may include a digital signal processor (DSP), a fieldprogrammable gate array (FPGA), a microprocessor, etc. In variousimplementations, additional components may be included in the IC 280.Additionally, various functions shown inside the IC 280 in FIG. 2 may beimplemented external to the IC 280, such as in a second IC or indiscrete circuitry. For example only, the supervisor control module 270may be integrated with the motor control module 260.

FIG. 3 a is a schematic of an example implementation of the PFC module204. The PFC module 204 receives AC power via first and second AC inputterminals 302 and 304. The AC power may be, for example, the AC poweroutput by the EMI filter 202. In various implementations, the signals atthe first and second AC input terminals 302 and 304 may both betime-varying with respect to an earth ground. The PFC module 204 outputsDC power to the DC filter 206 and the inverter power module 208 via apositive DC terminal 306 and a negative DC terminal 308.

An anode of a first rectifier diode 310 is connected to the second ACinput terminal 304, and a cathode of the first rectifier diode 310 isconnected to the positive DC terminal 306. An anode of a secondrectifier diode 312 is connected to the negative DC terminal 308, and acathode of the second rectifier diode 312 is connected to the second ACinput terminal 304. Each of the rectifier diodes 310 and 312 may beimplemented as one or more individual series or parallel diodes.

A switch block 320 is connected between the positive and negative DCterminals 306 and 308. The switch block 320 includes a first PFC leg 330that includes first and second switches 332 and 334. The switches 332and 334 each include a first terminal, a second terminal, and a controlterminal. In various implementations, each of the switches 332 and 334may be implemented as an insulated gate bipolar transistor (IGBT). Insuch implementations, the first, second, and control terminals maycorrespond to collector, emitter, and gate terminals, respectively.

The first terminal of the first switch 332 is connected to the positiveDC terminal 306. The second terminal of the first switch 332 isconnected to the first terminal of the second switch 334. The secondterminal of the second switch 334 may be connected to the negative DCterminal 308. In various implementations, the second terminal of thesecond switch 334 may be connected to the negative DC terminal 308 via ashunt resistor 380 to enable measuring current flowing through the firstPFC leg 330.

The control terminals of the switches 332 and 334 receive generallycomplementary PFC PWM signals from the PFC control module 250. In otherwords, the PFC PWM signal provided to the first switch 332 is oppositein polarity to the PFC PWM signal provided to the second switch 334.Short circuit current may flow when the turning on of one of theswitches 332 and 334 overlaps with the turning off of the other of theswitches 332 and 334. Therefore, both the switches 332 and 334 may beturned off during a deadtime before either one of the switches 332 and334 is turned on. Therefore, generally complementary means that twosignals are opposite for most of their periods. However, aroundtransitions, both signals may be low or high for some overlap period.

The first PFC leg 330 may also include first and second diodes 336 and338 connected anti-parallel to the switches 332 and 334, respectively.In other words, an anode of the first diode 336 is connected to thesecond terminal of the first switch 332, and a cathode of the firstdiode 336 is connected to the first terminal of the first switch 332. Ananode of the second diode 338 is connected to the second terminal of thesecond switch 334, and a cathode of the second diode 338 is connected tothe first terminal of the second switch 334.

The switch block 320 may include one or more additional PFC legs. Invarious implementations, the switch block 320 may include one additionalPFC leg. As shown in FIG. 3 a, the switch block 320 includes second andthird PFC legs 350 and 360. The number of PFC legs included in theswitch block 320 may be chosen based on performance and cost. Forexample only, the magnitude of ripple (voltage and current) in the DCoutput of the PFC module 204 may decrease as the number of PFC legsincreases. In addition, the amount of ripple current in the AC linecurrent may decrease as the number of PFC legs increase. However, partscosts and implementation complexity may increase as the number of PFClegs increases.

The second and third PFC legs 350 and 360 of the switch block 320 may besimilar to the first PFC leg 330. For example only, the second and thirdPFC legs 350 and 360 may each include respective components for theswitches 332 and 334, the diodes 336 and 338, and respective shuntresisters connected in the same manner as the first PFC leg 330.

The PFC PWM signals provided to the switches of the additional PFC legsmay also be complementary in nature. The PFC PWM signals provided to theadditional PFC legs may be phase shifted from each other and from thePFC PWM signals provided to the first PFC leg 330. For example only, thephase shift of the PFC PWM signals may be determined by dividing 360degrees)(° by the number of PFC legs. For example, when the switch block320 includes three PFC legs, the PFC PWM signals may be phase shiftedfrom each other by 120° (or 180° for two phases, or 90° for four phases,etc.). Phase shifting the PFC PWM signals may cancel ripple in the ACline current as well as the DC output.

The PFC module 204 includes a first inductor 370. The first inductor 370is connected between the first AC input terminal 302 and the secondterminal of the first switch 332. Additional inductors may connect thefirst AC input terminal 302 to additional PFC legs. For example only,FIG. 3 a shows a second inductor 372 and a third inductor 374 connectingthe first AC input terminal 302 to the second and third PFC legs 350 and360, respectively.

A voltage may be measured across the shunt resistor 380 to determinecurrent through the first PFC leg 330 according to Ohm's law. Anamplifier (not shown), such as an operational amplifier, may amplify thevoltage across the shunt resistor 380. The amplified voltage may bedigitized, buffered, and/or filtered to determine the current throughthe first PFC leg 330. Current through other PFC legs may be determinedusing respective shunt resistors.

Additionally or alternatively, a resistor 382 may be connected in serieswith the negative DC terminal 308, as shown in FIG. 3 b. Current throughthe resistor 382 may therefore indicate a total current output from thePFC module 204. Current through each of the PFC legs 330, 350, and 360may be inferred from the total current based on the known phase timingof the current through the PFC legs 330, 350, and 360.

Any method of measuring or sensing current through any or all of the PFClegs 330, 350, 360 may be used. For example, in various implementations,the current through the first PFC leg 330 may be measured using acurrent sensor 387 (as shown in FIG. 3 c). For example only, the currentsensor 387 may be implemented in series with the first inductor 370. Invarious implementations, the current sensor 387 may include aHall-effect sensor that measures the current through the first PFC leg330 based on magnetic flux around the first inductor 370. Currentthrough the PFC legs 350 and 360 may also be measured using associatedcurrent sensors 388 and 389, respectively.

The PFC module 204 may also include first and second bypass diodes 390and 392. An anode of the first bypass diode 390 is connected to thefirst AC input terminal 302, and a cathode of the first bypass diode 390is connected to the positive DC terminal 306. An anode of the secondbypass diode 392 is connected to the negative DC terminal 308, and acathode of the second bypass diode 392 is connected to the first ACinput terminal 302.

The bypass diodes 390 and 392 may be power diodes, which may be designedto operate at low frequencies, such as, for example, frequencies lessthan approximately 100 Hz or approximately 200 Hz. Resistance of thebypass diodes 390 and 392 may be less than resistance of the inductors370, 372, and 374. Therefore, when the switches 332 and 334 within theswitch block 320 are not being switched, current may flow through thebypass diodes 390 and 392 instead of the diodes 336 and 338.

When the PFC module 204 is operating to create a boosted DC voltage, theboosted DC voltage will be greater than a peak voltage on the AC line.The bypass diodes 390 and 392 will therefore not be forward biased andwill remain inactive. The bypass diodes 390 and 392 may providelightning strike protection and power surge protection.

In various implementations, the bypass diodes 390 and 392 may beimplemented with the rectifier diodes 310 and 312 in a single package.For example only, Vishay model number 26MT or 36MT or InternationalRectifier, model number 26 MB or 36 MB may be used as the bypass diodes390 and 392 and the rectifier diodes 310 and 312. The rectifier diodes310 and 312 carry current whether the PFC module 204 is generating aboosted DC voltage or not. Therefore, in various implementations, eachof the rectifier diodes 310 and 312 may be implemented as two physicaldiodes connected in parallel. Current sensors may be used to measure PFCphase currents in series with the inductors 370, 372, and 374.

Referring now to FIG. 4 a, a simplified schematic of a motor 400 and anexample implementation of the inverter power module 208 is presented.The motor 400 is a component of the compressor 102 of FIG. 2. However,the principles of FIGS. 4 a-4 c may apply to other motors, including amotor of the condenser 104. The inverter power module 208 includes aswitch block 402. In various implementations, the switch block 402 andthe switch block 320 of the PFC module 204 may be implemented using asimilar part. For example only, in FIG. 4 a, a first inverter leg 410includes first and second switches 420 and 422 and first and seconddiodes 424 and 426, which are arranged similarly to the switches 332 and334 and the diodes 336 and 338 of FIG. 3 a.

The switch block 402 receives the filtered DC voltage from the DC filter206 via a positive DC terminal 404 and a negative DC terminal 406. Thefirst terminal of the first switch 420 may be connected to the positiveDC terminal 404, while the second terminal of the second switch 422 maybe connected to the negative DC terminal 406. The control terminals ofthe switches 420 and 422 receive generally complementary inverter PWMsignals from the motor control module 260.

The switch block 402 may include one or more additional inverter legs.In various implementations, the switch block 402 may include oneinverter leg for each phase or winding of the motor 400. For exampleonly, the switch block 402 may include second and third inverter legs430 and 440, as shown in FIG. 4 a. The inverter legs 410, 430, and 440may provide current to windings 450, 452, and 454 of the motor 400,respectively. The windings 454, 452, and 450 may be referred to aswindings a, b, and c, respectively. Voltage applied to the windings 454,452, and 450 may be referred to as Va, Vb, and Vc, respectively. Currentthrough the windings 454, 452, and 450 may be referred to as Ia, Ib, andIc, respectively.

For example only, first ends of the windings 450, 452, and 454 may beconnected to a common node. Second ends of the windings 450, 452, and454 may be connected to the second terminal of the first switch 420 ofthe inverter legs 410, 430, and 440, respectively.

The inverter power module 208 may also include a shunt resistor 460 thatis associated with the first inverter leg 410. The shunt resistor 460may be connected between the second terminal of the second switch 422and the negative DC terminal 406. In various implementations, respectiveshunt resistors may be located between each of the inverter legs 430 and440 and the negative DC terminal 406. For example only, current throughthe first winding 450 of the motor 400 may be determined based on thevoltage across the shunt resistor 460 of the first inverter leg 410. Invarious implementations, the shunt resistor of one of the inverter legs410, 430, or 440 may be omitted. In such implementations, current may beinferred based on the measurements of the remaining shunt resistors.

Additionally or alternatively, a resistor 462 may be connected in serieswith the negative DC terminal 406, as shown in FIG. 4 b. Current throughthe resistor 462 may therefore indicate a total current consumed by theinverter power module 208. Current through each of the inverter legs410, 430, and 440 may be inferred from the total current based on theknown phase timing of the current through the inverter legs 410, 430,and 440. Further discussion of determining currents in an inverter canbe found in commonly assigned U.S. Pat. No. 7,193,388, issued Mar. 20,2007, which is incorporated by reference herein in its entirety.

Any method of measuring or sensing current through any or all of theinverter legs 410, 430, and 440 may be used. For example, in variousimplementations, the current through the first inverter leg 410 may bemeasured using a current sensor 487 (shown in FIG. 4 c). For exampleonly, the current sensor 487 may be implemented between the firstinverter leg 410 and the first winding 450. Current through the inverterlegs 430 and 440 may also be measured using associated current sensors488 and 489, respectively. In various implementations, current sensorsmay be associated with two of the inverter legs 410, 430, and 440. Thecurrent through the other one of the inverter legs 410, 430, and 440 maybe determined based on an assumption that the current in the motorwindings sums to zero.

Referring now to FIG. 5, a diagram of an example implementation of acommon DC bus refrigeration system 500 is presented. In someimplementations, the DC power from the PFC module 204 may also beprovided to the condenser 104. In various implementations, the DC powermay be filtered by the DC filter 206. Here, the DC bus from the DCfilter 206 is explicitly shown as including a positive DC line 502 and anegative DC line 504. Second positive and negative DC lines 506 and 508are connected between the DC lines 502 and 504, respectively, and acondenser inverter module 510.

The condenser inverter module 510 converts the DC power into AC powerthat is provided to the motor associated with the condenser 104 (e.g.,the condenser fan motor). The condenser fan motor may be referred to asthe condenser motor. In various implementations, the condenser invertermodule 510 may convert the DC power into three-phase AC power andprovide the three phases of the AC power to three respective windings ofthe condenser motor. The condenser inverter module 510 may convert theDC power into more or fewer phases of power. In various implementations,the condenser inverter module 510 may be similar or identical to theinverter power module 208.

A condenser motor control module 530 controls the condenser invertermodule 510. More specifically, the condenser motor control module 530controls the flow of power to the condenser motor. The condenser motorcontrol module 530 may control switches in the condenser inverter module510 using PWM in order to achieve a commanded condenser speed. The dutycycle of PWM signals applied to the condenser inverter module 510controls current through the windings of the condenser motor. Thecurrents control torque, and the condenser motor control module 530 maycontrol the torque to achieve the commanded condenser speed. As thecondenser inverter module 510 draws DC power from the DC bus, the PFCcontrol module 250 may control the PFC PWM signals to account for theoperation of the condenser inverter module 510 and the condenser motor.

The condenser motor control module 530 may be implemented independentlyof the IC 280 or may be implemented with components of the IC 280 in acommon IC, such as within a compressor/condenser IC 550. In variousimplementations, the condenser motor control module 530 may receive thecommanded condenser speed from the supervisor control module 270 or fromthe system controller 130. In various implementations, the commandedcondenser speed may be provided by the user interface 134 of FIG. 1.

FIG. 6 is a diagram of an example common DC bus refrigeration system600. Compared to the common DC bus refrigeration system 500 of FIG. 5,the refrigeration system 600 includes a condenser rectifier module 602.The condenser rectifier module 602 receives AC power, such as via the ACline output from the EMI filter 202. The condenser rectifier module 602rectifies the AC power, thereby converting the AC power into a second DCpower.

The condenser rectifier module 602 may include a full-bridge rectifierand may include circuitry to provide passive or active power factorcorrection. In various implementations, the condenser rectifier module602 may be similar or identical to the PFC module 204. A condenserrectifier control module 604 may be provided to control the condenserrectifier module 602. The condenser rectifier module 602 provides thesecond DC power to a condenser inverter module 610 via positive andnegative DC lines 612 and 614.

The condenser inverter module 610 converts the second DC power into ACpower that is provided to the condenser motor. A first connecting line615 connects the positive DC line 502 with the positive DC line 612. Asecond connecting line 616 connects the negative DC line 504 with thenegative DC line 614.

A diode 618 may be included in series with the first connecting line 615to block current from flowing from the positive DC line 612 to thepositive DC line 502. An anode of the diode 618 may be connected to thepositive DC line 502 and a cathode of the diode 618 may be connected tothe positive DC line 612. Power that may otherwise be fed back to thePFC module 204 when the compressor motor slows or is back-driven mayinstead be distributed to the condenser motor and/or the condenserrectifier module 602.

In various implementations, the condenser inverter module 610 mayconvert the second DC power into three-phase AC power and provide thephases of the AC power to three respective windings of the condensermotor. Alternatively, the condenser inverter module 610 may convert thesecond DC power into more or fewer phases of power. In variousimplementations, the condenser inverter module 610 may be similar oridentical to the inverter power module 208.

A condenser motor control module 630 controls the condenser invertermodule 610 and may operate similarly to the condenser motor controlmodule 530 of FIG. 5. The condenser motor control module 630 and thecondenser rectifier control module 604 may be implemented independentlyof the IC 280 or may be implemented with components of the IC 280 in acommon IC, such as within a compressor/condenser IC 650. In variousimplementations, the condenser motor control module 630 may receive thecommanded condenser speed from the supervisor control module 270 or fromthe system controller 130. In various implementations, the commandedcondenser speed may be provided by the user interface 134 of FIG. 1.

Typical PFC systems may receive a commanded fixed bus voltage. Thisfixed bus voltage, however, may be greater than is necessary to powerthe compressor 102, particularly in active PFC systems. The combinationof the excessive fixed bus voltage and power losses inherent to PFCoperation (as compared to passive/standard rectification) may result insignificant power losses. Further, low values of the fixed bus voltagemay cause the PFC system to switch on and off repeatedly, which mayresult in trips or faults. Under different operating conditions, thefixed bus voltage may be lower than is necessary to efficiently operatethe PFC system. More specifically, the fixed bus voltage may beinsufficient to operate the motor 400 at a desired speed under a highload.

Therefore, a system and method is presented that includes a variable busvoltage. More specifically, the system and method may determine adesired bus voltage (V_(DES)) based on one or more system parameters.For example only, V_(DES) may be controlled within a range of 355 Volts(V) to 410 V.

The system and method determines a commanded bus voltage (V_(BUS)) basedon V_(DES), and V_(BUS) is used to control operation of the PFC module204. When the PFC module 204 is turned on, the bus voltage is measuredand V_(BUS) is ramped from the measured bus voltage to a predeterminedstartup voltage during a predetermined startup period. The predeterminedstartup voltage may be chosen to stabilize the PFC module 204, toprevent damage of components, and/or to prevent trips/faults. Forexample only, the predetermined startup voltage may be 410 V, and thepredetermined startup period may be 15 seconds. After the predeterminedstartup period, V_(BUS) is controlled based on V_(DES) and V_(PEAK), asdescribed in more detail below.

Referring now to FIG. 7, an example bus voltage determination module 700is shown in more detail. In various implementations, the bus voltagedetermination module 700 may be implemented in the supervisor controlmodule 270. The bus voltage determination module 700 includes a voltagedetermination module 701, a bus voltage command module 704, a startupmodule 706, and a rate limiting module 708. The voltage determinationmodule 701 may include a look-up table 702.

The voltage determination module 701 receives a plurality of systemparameters. The voltage determination module 701 determines V_(DES)based on at least one of the plurality of system parameters. Theplurality of system parameters may include, for example only, actual andcommanded compressor speed, actual and estimated inverter output power,actual and estimated drive input power, input and output current,percentage out of volts (OOV), drive input voltage, inverter outputvoltage, estimated motor torque, a demand from the condenser 104, andvarious temperatures.

For example only, the various temperatures may include temperatures ofthe PFC module 204, the inverter power module 208, one or more circuitboards, a scroll of the compressor, and the compressor motor. Driveinput power is the electrical power flowing into the PFC module 204 asmeasured between the first and second AC input terminals 302 and 304(see FIG. 3 a). The drive input power can be measured using a powermeter with the line input current and voltage measured between the firstand second AC input terminals 302 and 304 as the two inputs to themeter.

The inverter output power is measured at the 3 drive output terminals ofthe inverter power module 208 (see FIG. 4 a). The inverter output powercan be determined by measuring each phase current (Ia, Ib, and Ic) andeach line to line voltage (Va-Vb, Vb-Vc, and Vc-Va). The differencebetween the inverter output power (power going to the motor 400) and thedrive input power (power entering the PFC module 204) represents thepower consumed by the PFC module 204 and the inverter power module 208.

For example only, as power (e.g., actual and estimated inverter outputpower, actual and estimated drive input power) increases, V_(DES) may beincreased or decreased. As current (e.g., input current, output current)decreases, V_(DES) may be increased or decreased. As line voltage (e.g.,drive input voltage and inverter output voltage) decreases, V_(DES) maybe decreased. As motor speed (e.g., actual and commanded compressorspeed and percentage OOV) increases, V_(DES) may be increased. As torque(e.g., motor torque in the compressor 102) increases, V_(DES) may beincreased. As selected ones of the various temperatures decrease,V_(DES) may be increased. Furthermore, changes in any combination of theabove described parameters may affect V_(DES).

The look-up table 702 may store predetermined relationships betweenV_(DES), AC peak voltage V_(PEAK), and combinations of the plurality ofsystem parameters. The look-up table 702 may include data correspondingto a predetermined range of V_(DES). For example only, the predeterminedrange for V_(DES) may be 355 V-410 V. The look-up table 702 may alsoinclude data corresponding to additional values of V_(DES).

The bus voltage command module 704 receives V_(PEAK), the peak voltageof the AC line signal. The peak voltage of the AC line signal may bedetermined by simply monitoring the voltage of the AC line signal (suchas by periodic digital sampling) and selecting the highest voltage asthe peak voltage. However, this method may be susceptible to noise andother transients, which may cause the measured peak voltage to beartificially high. Alternatively, V_(PEAK) may be determined bymultiplying a mean absolute value of the AC line signal by π/2. The meanabsolute value of the AC line signal is much less susceptible to noiseand other transients. V_(PEAK) may be determined at predeterminedintervals, such as once per AC line cycle.

The bus voltage command module 704 determines V_(BUS) based on V_(DES)from the voltage determination module 701. As discussed further below,the bus voltage command module 704 may adjust V_(BUS) based on one ormore other parameters, such as V_(PEAK), V_(HOLD), and the measured busvoltage.

When the PFC module 204 is off, the measured bus voltage may be lessthan V_(PEAK) because of passive operation of diodes within the PFCmodule 204. After the PFC module 204 is initially turned on, the startupmodule 706 generates a start signal having a first state (e.g., high or“1”). The startup module 706 may maintain the start signal at the firststate for a predetermined startup period (t_(START)). For example only,t_(START) may be approximately 15 seconds. The start signal is sent tothe bus voltage command module 704. To avoid a discontinuity, the busvoltage command module 704 may set V_(BUS) to the measured bus voltagewhen the start signal having the first state is received.

The start signal may also be sent to the rate limiting module 708. Therate limiting module 708 may generate a limited commanded bus voltage byapplying a rate limit to V_(BUS) from the bus voltage command module704. However, when the rate limiting module 708 receives the startsignal having the first state, the rate limiting module 708 initializesthe limited commanded bus voltage to V_(BUS), which was set based on themeasured bus voltage. After initializing the limited commanded busvoltage to V_(BUS), the rate limiting module 708 returns to generatingthe limited commanded bus voltage by applying a rate limit to changes inV_(BUS).

The limited commanded bus voltage is used to control the PFC module 204.For example only, the rate limiting module 708 may output the limitedcommanded bus voltage to the PFC control module 250. The rate limitingmodule 708 may implement the rate limiting by adjusting the limitedcommanded bus voltage toward V_(BUS) after each time interval of aspecified length. The amount by which the limited commanded bus voltagecan change during each time interval is limited to a specifiedincrement. The average rate applied by the rate limiting module 708 isthen a ratio of the specified increment to the specified length.

The rate applied by the rate limiting module 708 may be asymmetric—witha higher rate in one direction than another (e.g., decreasing is limitedto a higher rate than increasing). In various implementations, the ratelimiting may be non-linear.

After beginning to generate the start signal having the first state, thestartup module 706 may provide a startup voltage V_(START) to the busvoltage command module 704. V_(START) may be chosen as a minimum voltagethat will create stable start conditions for the PFC module 204. Forexample only, V_(START) may be approximately 410 V. While the startsignal remains in the first state, the bus voltage command module 704sets V_(BUS) to be equal to V_(START).

Because the rate limiting module 708 applies a rate limit, the limitedcommanded bus voltage begins ramping to the new value of V_(BUS),V_(START). For example only, if V_(START) is 410 V, and the measured busvoltage is 325 V, the rate limiting module 708 may ramp the limitedcommanded bus voltage from 325 V to 410 V.

After the predetermined startup period t_(START), the startup module 706transitions the start signal to a second state (e.g., low, or “0”). Whenthe start signal has the second state, the bus voltage command module704 begins to control V_(BUS) based on V_(DES).

The bus voltage command module 704 may apply a lower limit to V_(DES)when determining V_(BUS). The PFC module 204 may be configured to boostthe DC bus voltage to greater than V_(PEAK). For example only, the PFCmodule 204 may be able to maintain a limited commanded bus voltage thatis greater than V_(PEAK) plus an offset voltage.

By contrast, the PFC module 204 may not be able to produce a limitedcommanded bus voltage that is less than the offset voltage plusV_(PEAK). To produce such a limited commanded bus voltage, the PFCmodule 204 may be switched off and on. Switching the PFC module 204 offand on may create unstable conditions, and result in trips or faults.

Therefore, when determining V_(BUS), the bus voltage command module 704may apply a lower limit that is equal to V_(PEAK) plus the offsetvoltage. For example only, the offset voltage may be approximately 30 V.In other words, the bus voltage command module 704 may increase V_(BUS)to the lower limit when the lower limit is greater than V_(BUS). The busvoltage command module 704 also increases V_(BUS) to the value ofV_(DES) when V_(DES) is greater than V_(BUS).

The bus voltage command module 704 may prevent a reduction in V_(BUS)unless a predetermined period has passed since V_(BUS) was lastincreased. Further, at the end of the predetermined period, the busvoltage command module 704 may determine the lower limit based on notthe current value of V_(PEAK), but the highest value of V_(PEAK)observed within the predetermined period. This prevents prematurelydecreasing V_(BUS) when an unusually low value of V_(PEAK) was observedat the end of the predetermined period. For example only, thepredetermined period may be approximately 10 seconds.

Referring now to FIG. 8, a flow diagram depicts example operation of thebus voltage determination module 700. Control begins at 804, wherecontrol sets V_(BUS) equal to the measured bus voltage. Control thenenables rate limiting of V_(BUS) at 808. When rate limiting is enabled,control applies a rate limit to changes in V_(BUS) and outputs theresult as a limited commanded bus voltage.

At 812, control sets V_(BUS) equal to a predetermined startup voltageV_(START). At 816, control waits for a predetermined startup periodt_(START). For example only, t_(START) may be approximately 10 seconds,and V_(START) may be approximately 410 V. As stated above, control ratelimits the transition of V_(BUS) from the measured bus voltage toV_(START).

Control continues at 820 and sets a peak hold voltage V_(HOLD) equal tothe current peak AC voltage V_(PEAK). At 824, control initializes atimer to zero, which allows the timer to track a time period elapsedsince the timer was last initialized. At 828, control determines thedesired bus voltage V_(DES) based on one or more system parameters.

At 830, control determines whether: (1) V_(BUS) is less than a sum ofV_(PEAK) and an offset voltage; and/or (2) V_(BUS) is less than V_(DES).If either of these conditions is true, control transfers to 832. If bothof the conditions are false, control continues to 848.

At 832, control determines whether V_(BUS) is less than the sum ofV_(PEAK) and the offset voltage. If true, control sets V_(BUS) equal tothe sum of V_(PEAK) and the offset voltage in 836 and continues to 840;otherwise, control transfers to 840.

At 840, control determines whether V_(BUS) is less than V_(DES). Iftrue, control sets V_(BUS) equal to V_(DES) at 844 and returns to 820;if false, control simply returns to 820. In this manner, controlincreases V_(BUS) and resets the timer when V_(BUS) is less than eitherV_(DES) or the sum of V_(PEAK) and the offset voltage.

At 848, control determines whether the timer is greater than apredetermined period. If true, control transfers to 852; if false,control continues to 854. For example only, the predetermined period maybe approximately 10 seconds. At 854, control determines whether V_(PEAK)is greater than V_(HOLD). If true, control updates V_(HOLD) to be equalto V_(PEAK) at 868 and returns to 828; if false, control simply returnsto 828. In this manner, V_(HOLD) tracks the highest V_(PEAK) observedsince V_(HOLD) was initialized at 820.

At 852, control determines whether V_(DES) is less than a sum ofV_(HOLD) and the voltage offset. If true, control sets V_(BUS) equal tothe sum of V_(HOLD) and the offset voltage at 858 and returns to 820; iffalse, control sets V_(BUS) equal to V_(DES) at 860 and returns to 820.In other words, each time the predetermined period expires (as measuredby the timer at 848), V_(BUS) can be lowered to the greater of V_(DES)and the sum of V_(HOLD) (the highest V_(PEAK) observed within thatpredetermined period) and the offset voltage. The predetermined periodmay be selected to be long enough that V_(HOLD) is relatively steadywhile not maintaining V_(HOLD) at an artificially high level for toolong.

The broad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification, and the following claims.

What is claimed is:
 1. A controller comprising: a bus voltage commandmodule that: (i) outputs a commanded bus voltage for a direct current(DC) bus electrically connected between a power factor correction (PFC)module and an inverter power module that drives a motor; (ii) sets thecommanded bus voltage equal to a measured voltage of the DC bus when thecontroller transitions from an off state to an on state; and (iii) setsthe commanded bus voltage equal to a predetermined startup voltage for apredetermined startup period after the controller transitions from theoff state to the on state; a rate limiting module that ramps a ratelimited voltage toward the commanded bus voltage during thepredetermined startup period based on a predetermined rate limit; and aPFC control module that controls the PFC module to create a voltage onthe DC bus that is based on the rate limited voltage.
 2. A methodcomprising: generating, using a controller, a commanded bus voltage fora direct current (DC) bus electrically connected between a power factorcorrection (PFC) module and an inverter power module that drives amotor; setting, using the controller, the commanded bus voltage equal toa measured voltage of the DC bus when the controller transitions from anoff state to an on state; setting, using the controller, the commandedbus voltage equal to a predetermined startup voltage for a predeterminedstartup period after the controller transitions from the off state tothe on state; ramping, using the controller, a rate limited voltagetoward the commanded bus voltage during the predetermined startup periodbased on a predetermined rate limit; and controlling, using thecontroller, the PFC module to create a voltage on the DC bus that isbased on the rate limited voltage.
 3. A method comprising: generating acommanded bus voltage for a direct current (DC) bus electricallyconnected between a power factor correction (PFC) module and an inverterpower module that drives a motor; setting the commanded bus voltageequal to a measured voltage of the DC bus upon controller power-off;setting the commanded bus voltage equal to a predetermined startupvoltage for a predetermined startup period upon controller power-on;ramping a rate limited voltage toward the commanded bus voltage duringthe predetermined startup period based on a predetermined rate limit;and controlling the PFC module to create a voltage on the DC bus that isbased on the rate limited voltage.